FIG. 1 is a frame structure of Time Division Duplex (TDD) mode in Long Term Evolution (LTE) system. In the frame structure, a 10 ms radio frame is divided into two half-frames, and each half-frame is divided into 10 time slots (number from 0 to 9) with a length of 0.5 ms respectively, and two time slots constitute a subframe with a length of 1 ms, and each half-frame consists of five subframes (number from 0 to 4). Wherein, subframe 0 is fixedly used for downlink, and subframe 1 is a special subframe containing 3 particular time slots, which are Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS) receptively. The former n subframes after subframe 1 are used for uplink transmission (1≦n≦3), and the latter 3-n subframes are used for downlink transmission. Random access channel may be transmitted either in the UpPTS or in other uplink subframes, but the structures of this two random access channels are different.
In the LTE system, Random Access Channel (RACH) uses cyclic shift sequences of Zadoff-Chu (ZC) sequence as preamble, and these cyclic shift sequences can also be called Zero Correlation Zone (ZCZ) sequences.
In the LTE system, mobile phone first executes downlink synchronization after being turned on, and then begins to detect Broadcast Channel (BCH). Base station informs the mobile phone via the BCH about the available index of the first ZC sequence and step size of cyclic shift (i.e. cyclic shift amount) of the RACH in the cell, and the mobile phone uses a certain mapping rule to calculate sequence numbers of corresponding ZC sequences according to the index, and then generates available ZCZ sequences according to cyclic shift step size and a certain cyclic shift restriction rule. If the amount of the ZCZ sequences is less than a threshold M, the mobile phone increases sequence index by degrees automatically, and uses the next ZC sequence to continue generating ZCZ sequences until the total amount of ZCZ sequences is greater than or equal to said threshold. In the end, the mobile phone randomly selects one sequence from all generated available ZCZ sequences as a random access preamble (hereinafter referred to as preamble for short) to send.
The selection of cyclic shift amount (Ncs) affects system performances. If the cyclic shift amount is too great, the amount of the ZCZ sequences generated by each ZC sequence will become smaller, so that the reuse factor of ZC sequence decrease (reuse factor is defined as amount of cells using different ZC sequences), and the interference between cells increase; if the cyclic shift amount is too small, the supported covering range will become too small to satisfy the demond of networking. Furthermore, unreasonable design of the cyclic shift amount will also decrease the complete orthogonal probability of two preambles and increase the interference between users in cells.
Therefore, it is necessary to design a set of reasonable cyclic shift amount to satisfy maximal covering demand of the system, furthermore, under the condition of different covering demands, a suitable cyclic shift amount can be selected to make ZC sequence reuse factor reach the maximum and make two preambles reach the highest orthogonal probability. There has no related technical solution in existing technology yet.